Pigments, such as quinacridone, are well known and can be synthesized by the reaction which is illustrated below: ##STR1##
By using amines other than aniline, substituted quinacridones, also having pigmentary characteristics, can be prepared.
As shown above, the last step in the reaction is a ring closing reaction with polyphosphoric acid. The by-product of that step is a phosphoric acid waste stream contaminated with organic materials. Past attempts at purifying the waste phosphoric acid for reuse have met with difficulties.
The purified phosphoric acid that is obtained remains contaminated with organic materials. Heretofore, the only outlet for this waste phosphoric acid has been as fertilizer.